Minneapolis Water Works Ultrafiltration Plant Makes a Big Splash in the Membrane World

The quality of drinking water in the Minneapolis area has been significantly enhanced by a new membrane filtration plant now in operation in Columbia Heights. The largest potable ultrafiltration (UF) plant in the Western Hemisphere, which is also the second largest in the world, is now treating and supplying high-quality drinking water to area residents. With a current capacity of 70 million gallons per day (mgd) and ultimate capacity of 78 mgd, the Minneapolis Water Works' Columbia Heights membrane filtration plant is second in size only to a 72 mgd membrane facility in Singapore. Both facilities were designed by Black & Veatch.

Growing interest in membrane filtration by communities around the world has spawned an increase in not only the number but also the size and sophistication of membrane facilities. With completion of the MWW UF plant, Black & Veatch has provided study, design, construction and startup services for the two largest membrane filtration facilities in the world treating potable water. The award-winning Chestnut Avenue Water Works extension project in Singapore features an immersed membrane system and continues to hold the world record in current operation capacity for potable water plants. The MWW plant further establishes membrane filtration as an economically viable technology. In fact, recent Black & Veatch projects in North America have shown membrane filtration to be not only affordable but even lower in cost than traditional filtration processes for Greenfield sites.

The decision to replace existing granular media filters with UF to further treat lime-softened, clarified, variable-quality water from the Mississippi River was prompted by multiple goals and considerations. These included the need to address future contamination threats, continue to comply with regulatory requirements and replace nearly century-old treatment sand filters. The new membrane filtration plant is producing low-turbidity water that is better than regulatory standards require, and it will protect MWW customers from waterborne disease by providing high-log removal of microbial pathogens such as Cryptosporidium.

Raw water is abstracted directly from the Mississippi River. The river receives numerous discharges in the watershed, including agricultural run off, power plant cooling water and effluent from sewage treatment plants. These threats have driven improvements to the treatment plant over the years. Notable treatment challenges include variable organic concentration, turbidity, and temperature. The annual snow melt period in the spring also creates treatment challenges for the pretreatment process due to rapid changes in raw water alkalinity and organic composition.

Previous study indicated that membrane filtration would provide better protection against contamination threats present in the watershed. Study team members also recognized that optimization of pretreatment would be necessary to successfully integrate membrane filtration. Such improvements included optimization of PAC dosing and use of potassium permanganate to control taste and odor.

Once the MWW decided to add membrane filtration, the utility prepared procurement documentation for the testing and purchase of filtration equipment. A panel of industry experts with membrane filtration technology was formed to provide input for the procurement documents and to review the pilot testing that accompanied the bidding process. Following pilot tests, extensive bench-scale tests, and a value engineering study, UF was selected as the most appropriate technology because it met the established criteria at an acceptable cost. Design of the Columbia Heights UF facility commenced in January 2001 following selection of Black & Veatch as the design engineer and Ionics (now part of GE Water Infrastructure) as the membrane equipment supplier.

At the Columbia Heights UF facility, the membrane filtration process is fed by five variable frequency-driven pumps. Water is distributed to four rows of membrane units; each row comprises nine ultrafiltration membrane units, each with a gross capacity of 2.2 mgd operating at a flux of 57 gfd (97 L/m2/h). Feed water to each membrane unit is pre-filtered by automatic backwash strainers of a candle filter design, which are periodically washed via a rotating wash arm. Strainers remain in service during backwashing, and each wash takes approximately two minutes.

The membrane units were manufactured by Ionics, Inc and use UF membrane cartridges manufactured by X-Flow, a Dutch membrane manufacturing company. Each membrane unit comprises 28 fiberglass pressure vessels similar to those used in reverse osmosis systems, 28 feet in length. Each pressure vessel contains four membrane cartridges connected in series, and bypass channels within each cartridge allow for even flow distribution throughout the pressure vessel.

Integration of new complex treatment into existing treatment facilities presents challenges, and the Columbia Heights UF plant was no exception. Challenges that necessitated special attention included flow control and hydraulic balancing through the existing pretreatment process to meet the demands of the ultrafiltration system; treatment, recovery, and recycle of waste streams such as chemical cleaning waste, backwash water from the membrane units, and feed strainers and integrity test drain down water; construction of new chemical feed processes to replace existing facilities while maintaining plant operations; and ensuring ease of access for anticipated future maintenance activities such as replacement or repair of valves and membrane fiber.

Extensive piloting was conducted throughout the procurement, design, and post-construction phase of this project. Initial pilot testing was performed as part of the procurement process to identify the most economical membrane system that met strict performance goals. Membrane manufacturers were required to establish operating parameters at the start of the initial piloting phase. At the end of the optimization period, the membrane suppliers had to identify their operating parameters and conduct extended piloting without any further adjustments. For the Columbia Heights project, two membrane systems made the shortlist following the initial advertisement. The systems piloted were Koch and Ionics, using the X-flow membrane. The Ionics system performed best and was ultimately selected for installation at the Columbia Heights facility. Additional piloting was conducted following selection to further optimize system parameters.

A major fouling event occurred during the extended piloting phase. It is believed to have coincided with a major change in water quality due to snow thawing in the spring. This causes softening of the Mississippi River water source, and it has been postulated that the nature of organic species may have been altered at this time. An alternative chemical cleaning procedure has been developed to restore permeability caused by the fouling event. After successful piloting, adjustments were made to the design of the chemical feed and neutralization systems. Chemical feed system changes are relatively inexpensive when caught during the design phase of the project but can be very expensive to modify once the plant has been constructed. Long-term pilot trials can be quite beneficial for utilities with rapidly changing surface water quality.

The MWW intends to continue pilot testing throughout the life of this plant. A demonstration scale unit is planned to enable utility staff to conduct off-line testing. Such testing may encompass alternative chemical cleaning regimes, additional virus testing, and alternative membrane formulations as they become available to compare performance with the existing plant.

The Columbia Heights UF plant project team incorporated advanced design tools as well as filtration technology in the upgrade of the existing water treatment facilities, using innovative 3 D design, virtual reality software, and an effective internet-based communications approach. While the exterior was designed to aesthetically complement the historic existing facilities, the new building houses contemporary control, laboratory and educational facilities in addition to the 40 UF units and associated storage and feed equipment for membrane cleaning.

Clearly identifying the best treatment solution and effectively communicating the need for rate increases to citizens and political leaders enabled the MWW to raise funds and increase rates as necessary. The professionals involved in the massive planning, design, construction, and implementation effort also concluded that it is beneficial to: continue piloting after bid award, especially if the water source is of changeable nature; allow for flexible chemical cleaning regimes to address unexpected fouling events; maintain and enforce rigid protocols for membrane pilot testing to gather high quality data to accurately determine the best performing membrane filtration unit; and maintain close working relationships among the owner, contractor, and design engineer. They also demonstrated that 3-D design is an excellent tool for developing a well laid-out plant that facilitates operations and maintenance work and careful coordination with regulatory agencies enables smooth passage through the approval process.

Jonathan Pressdee is an Associate Vice President and Membrane Practice Leader with B&V Water, the water business of Black & Veatch Corporation. Currently based in the company's Minneapolis office, Pressdee has 15 years experience in the water industry and holds a degree in Chemical Engineering from University College Swansea, University of Wales, UK. He can be reached at pressdeejr@bv.com.

Associate Vice President Chad Hill, P.E., served as the project director with Black & Veatch in Minneapolis. He has 19 years of project experience and is responsible for client services in upper midwestern United States. Hill earned his Masters degree from the University of Kansas, Lawrence, Kan.

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